This invention relates to a method of welding tubular members such as casings of a gear pump, tubular member welded by use of the welding method, and a gear pump including casings welded by use of the welding method.
FIGS. 2–4 show a gear pump assembly P which is disclosed in JP patent publications 2000-9058 and 2001-80498. It is mounted in a housing H of various devices, and sucks oil a through inlet ports 1 and discharge it through outlet ports 2. Specifically, the pump assembly comprises a rotary shaft 3 driven by a motor M, and a plurality of (two in the illustrated example) gear pumps arranged axially around the rotary shaft 3 and each comprising an inner rotor 4 fixed to the rotary shaft 3 and an outer rotor 5 rotatably mounted in the housing H so as to be eccentric relative to the inner rotor 4. The inner rotor 4 and the outer rotor 5 mesh with each other, thereby causing oil to be sucked into and discharged from the gear pump.
The pump assembly further includes casings 7 each housing a gear pump, a center cylinder 7a and side cylinders 7b which cooperate to define pump spaces in which are received the gear pumps. Each casing 6 has to be hermetically joined to the center cylinder 7a and the corresponding side cylinder 7b. Ordinarily, each casing 6 is welded to the cylinders 7a and 7b over the entire outer circumferential edges thereof as shown at t.
Such welding is started from a desired point of the outer circumferential edge of each casing 6 with a welder S facing the casing 6 and continued in a circumferential direction.
FIG. 3 shows the section of one of the gear pumps. As shown, the outer rotor 5 is biased by slide seals (apex seals) 8 such that gaps defined by the teeth of the inner rotor 4 and the outer rotor 5 gradually increase from the outlet port toward the inlet port. The seals 8 are received in recesses 9 formed in the casing 6. The wall of the casing 6 is thus thin at points where there are the recesses 9. During welding, solidifying force locally act on the casing 6. The condensing force tends to deform the casing 6 more markedly at its thin and thus weak portions due to the presence of the recesses 9 than at other portions.
For example, if welding is started at the point shown in FIG. 5, when welding is being carried out at the portion of a member 10 (casing 6) to be welded to other member where there is a cutout 11 (recess 9), due to the solidifying force b resulting from welding energy, bending stresses Q are applied to the member 10 (casing 6), thus deforming the member 10 into an oval shape of which the minor axis passes the cutout 11, as shown by chain line in FIG. 5.
On the other hand, if the member 10 has no cutout, when welding is started at a point c, due to the solidifying force produced at the welding start point c, the member will be deformed into an oval shape of which the minor axis passes the welding start point c, and the portions in the circumference will deform in a sinusoidal curve, as shown in FIG. 6.
If the member is a casing 6 of a gear pump P, it will be difficult to mount the outer rotor 5 in such a markedly deformed casing with high accuracy or may be utterly impossible to mount the outer rotor 5 in such a casing.
An object of this invention is to minimize the degree of deformation of such a member to be welded.